1. Field of the Invention
This invention relates to a method and device for managing the vehicle warm-up phase in a cold start situation by controlling a hydraulic retarder.
2. Description of the Prior Art
Some types of vehicles, especially, but not exclusively, industrial or commercial vehicles, are required to work in extremely low outdoor temperatures, where they may also remain out of service for a prolonged period of time; this may be the case, for example, in arctic conditions or wherever the temperature falls well below zero at night. In such conditions, when these vehicles are (re)started it takes them a long time to reach the steady state, especially as regards the temperature of the engine coolant, engine lubricating oil, transmission oil and the oil of the brake cooling system, in vehicles equipped with oil-bath brakes. The following advantages can be achieved by reducing warm-up times:                The vehicle is ready for use more quickly (in many automatic transmissions the gears cannot be selected if the oil is below a given temperature, which means that the vehicle cannot move). In particularly cold conditions, even if the engine idles for a prolonged period of time the engine coolant does not rise to above 50–60° C.;        Rapid defrosting of passenger compartment windows and achievement of the required temperature inside the cabin;        Less likelihood of jerky movements in a cold start situation, for example when the driver is impatient to get started;        Reduced wear on the engine, since this is not made to run for several tens of minutes with loose-fitting mechanical parts and less than optimal lubrication conditions;        Less engine fouling due to a reduction in the number of “misfiring” events that are due to the low temperature of the combustion chambers with the engine idling;        Reduction of atmospheric pollution during the warm-up phase as a consequence of the reduced number of “misfiring” events, which generate high levels of CO (carbon monoxide) and HC (unburnt hydrocarbons), both of which are extremely harmful.        
The use of electric resistance heaters to cut warm-up times is known in the prior art. This solution has often been used in the past in mechanical injection engines (that have more problems in cold start conditions than electronic injection engines), but it involves expensive adjustments to the vehicle in order to install the necessary components. Furthermore, problems may arise when connecting the resistances to the electric circuits, which may be outside the vehicle, and energy consumption is high.
In some industrial applications where the costs involved are particularly high and vehicles are required to work in extremely harsh environmental conditions, the braking system is often housed in a sealed oil bath to guarantee high braking torques while enhancing the vehicle's service life.
There may be two different fluids/circuits in the braking chambers:                the brake fluid, that flows under high pressure, up to and above 100 bar;        the brake cooling oil, which is separate from the brake fluid and ideally requires minimum backpressure levels (usually between 2 and 6 bar) that are strongly influenced by the operating temperature. The backpressure threshold depends on the type of seal that is used and prevents the risk of cross-contamination between the two oils. It is easy to maintain low backpressure levels under normal working conditions, with an oil temperature of approximately 80° C., but this becomes more difficult in cold start conditions, with outdoor temperatures of less than 5° C., due to the significant increase in the viscosity of the oil.        
In order to overcome this drawback the flow rate of the cooling oil must be reduced until this has warmed up sufficiently. During this time the brakes are not fully operational, in that only a part of the oil that is available is circulating. In this phase excessive use of the brakes could cause irreversible damage.
Hence reducing the time required for the oil in the brake cooling system to warm up should be beneficial in order to increase reliability and safety.